VINTAGE RADIO
By JOHN HILL
A pair of old AWA C79 chassis
Fixing vintage radio receivers for other
collectors can be quite a challenge. This is
the story of how a couple of old 5-valve TRF
console receivers were resurrected.
other had been recently worked on by
someone else.
For the purpose of this article I will
refer mostly to the near original set.
This one was worked on first so that I
could become familiar with it before
moving onto the modified one.
I recently met a vintage radio collector by the name of Dick Howarth.
Never have I come across a collector
with Dick’s enthusiasm. In a time
span of less than nine months (at the
time of writing), Dick has collected no
fewer than 15 early console radios with
turned legs. This style of receiver is
about the only type he is interested in.
There is little doubt that consoles
from the late 1920s and early 1930s
are very collectable items and to obtain
more than a dozen such receivers is
evidence of Dick’s enthusiasm. It is
not unusual for him to drive interstate
just on the off-chance of finding an
interesting old radio.
Capacitor blocks
Personally, I don’t care how many
radios Dick finds or where he finds
them. In fact, I hope that he keeps
on finding them. Why am I so supportive? Because Dick wants me to
do his repairs – that’s why! Not only
is it paying work, it also helps extend
my repair knowledge and supplies
me with interesting material for my
monthly column, as is the case this
month.
Two of Dick’s radios share similar
chassis. They are a 1930 AGE 44A
and a 1931 AWA 45E. Both use the
AWA C79 chassis and I had these
to repair at the same time. One was
almost completely original while the
The C79 chassis uses a considerable amount of paper capacitors. The capacitor
block on the right contains many individual units, wired together internally, &
has no less than 10 colour-coded lead-out wires.
70 Silicon Chip
The chassis that had recently been
worked on had already cost Dick
$150 and yet no attempt had been
made to replace the numerous paper
capacitors throughout the set. The
only capacitors replaced were three
15µF electrolytics.
Now some of these old receivers
are a bit daunting to work on if one is
unaccustomed to sets of this vintage.
Although the C79 is a relatively simple 5-valve TRF type receiver, it does
not look at all simple when you are
working on it.
One of the main problems is the
use of capacitor blocks which contain multiple units of bulky paper
capacitors. Along the front of the
chassis there are three metal cans,
each containing three 0.25µF 1000V
capacitors, while a much larger block
capacitor is housed under a pressed
steel cover mounted on top of the
chassis. This cover also houses a
tapped high-tension choke.
The big block capacitor is a bit of a
nightmare because there are 10 coloured leads coming from it that go to
various connection points throughout
the circuit. One has to be careful when
disconnecting this block capacitor
because it has to be duplicated with
modern capacitors and reconnected as
it was originally.
It is necessary to make an accurate
sketch showing which coloured wires
go where. As there are two black, two
light brown and three blue leads,
one must be attentive. The fact that
This large pressed steel can houses the tapped high tension filter choke (still in
the can) & a huge block capacitor.
the colours have faded doesn’t help
either.
Block capacitors have been dealt
with in a previous story. It should be
sufficient to say that, in the case of
these two C79 chassis, the capacitors
were more than 60 years old and it is
unreasonable to expect them to still be
in working order. They were replaced
without hesitation!
Tuning setup
These two old radios from the early
1930s have other odd characteristics
apart from the huge wad of paper capacitors. Following is a brief description of some other notable aspects of
the C79.
The tuning setup is unusual in
that it uses three single tuning capacitors which are interconnected
(ganged) by a network of steel belts
and pulleys.
To find mechanically coupled independent tuning capacitors in 1930-31
receivers was a surprise. I had been
under the impres­sion that that idea
had gone out of fashion several years
before.
The valve line up for the C79 is
fairly standard for the era and consists
of three 24As, a 45 output and the
usual 80 rectifier. The 24A is a radio
frequency (RF) tetrode, while the 45 is
a directly heated output triode.
The volume is controlled by a
This metal can contains three 0.25µF paper capacitors.
After 64 years, the wax that sealed the capacitors from at­
mospheric moisture had shrunk away from the can & had
become useless.
wirewound potentiometer which
varies the screen voltage on the RF
valves. This technique differs from
the more usual cathode bias arrangement. The volume control was in good
condition.
Another oddity is the electrodynamic loudspeaker. First the speaker
cone has a soft leather outer rim
suspension to give it flexibility. And
second, the field coil has a fairly
high im­pedance of 7500 ohms and
is placed directly across the high
tension supply.
It is unusual items such as the
C79’s speaker that makes repairs to
these early AC receivers fairly difficult –especially if the set is to remain
reasonably original.
As far as I’m concerned, hard to
find spare parts are Dick’s problem.
He has to chase around and locate
these out-of-the-ordinary bits and
pieces – not me.
While both C79 chassis are driving
speakers of the original type, one of
them came from Queensland and it
was possibly sheer good luck that it
was found when it was needed. Usable
spares in good condition are becoming
quite difficult to find.
The tapped high-tension choke has
been previously mentioned and, as
luck would have it, both chassis had
their chokes intact. There is also a
much smaller choke mounted at one
end of the chassis in a pressed steel
can. There are two such cans actually;
one houses the choke while the other
The paper capacitors have been replaced with modern
polyester types. Faulty capacitors cause many problems
and their replace­ment is a logical step in the restoration of
old radio receivers.
November 1994 71
This front view of chassis shows the dial & one of the two drive belts & pulleys
used to interconnect the three separate tuning capacitors. The tone switch is in
the foreground.
popularity, prior to the universal
acceptance of re­sistance-capacitance
coupling. The aim was to provide the
highest practical plate voltage for the
driving valve, while still pro­viding a
high value plate load. This would be
particularly im­portant considering the
high drive requirements of the type
45 valve. Subsequently, higher gain
output valves made the more economical resistance coupled system a better
proposition).
The open circuit output transformer
could not be repaired and a replacement was installed. As the new unit
was a little smaller than the original,
the 3-piece steel can that housed it
had to be held together with an epoxy
resin adhesive (Araldite®), otherwise
it would have fallen apart. The original
output trans­former filled the can and
held the three pieces in place.
Switching on
The 80 rectifier valve sits between the coupling choke on the left & the output
transformer on the right. Both these units were open circuit. The transformer
was replaced, while a 0.5MΩ resistor was connected across the choke terminals.
contains the output trans­
former. In
one chassis, both units were open
circuit while in the other chassis they
were OK.
Incidentally, this small chassis-mounted choke is used as a coupling device (in conjunction with
a capacitor) to couple the detector
valve to the output valve. It serves as a
plate load for the detector. A previous
repairer had inserted a 100kΩ resistor
across the open choke connections
which seems a logical and easy way
out of the problem. However, it was
discovered later, when the set was
working, that a 0.5MΩ resistor gave
much better results.
72 Silicon Chip
It seems to me that many of these
early AC-powered receiv­ers were more
complicated than they really needed
to be. In later years, all these elaborate
and expensive chokes were removed
from radio circuits. The field coil alone
served the dual role of a high tension
choke and an electromagnet for the
loudspeaker – without the need for
additional chokes in the high tension.
As previously stated, a 0.5MΩ resistor was an adequate replacement
for the open circuit coupling choke;
so there wasn’t much need for that
particular component!
(Editorial note: choke-capacitance
coupling enjoyed a brief period of
After many hours of work, the receiver was finally ready for a tryout.
Two seconds after switch on, it was
apparent that all was not well. A
dreadful loud buzzing sound was all
that could be heard through the loudspeaker – not the sound I expected
to hear.
Now this speaker was the one that
came from Queensland and I had been
told that it was in good working order.
It had been plugged into another working set and it functioned quite OK, so
I had no reason to doubt it.
But what I wasn’t told was that the
speaker plug had been removed and
the pin connections resoldered but not
checked. As a result, the plug leads
had been inserted into the wrong pins,
resulting in the voice coil being wired
across the high tension supply. No
wonder it made such a noise!
It took quite some time to work out
what the problem was, for the simple
reason that I had been told that the
speaker was OK. It was only after trying another loudspeaker that I realised
what was wrong.
There is a good lesson to be learnt
there. Don’t believe anything anyone
tells you until you have checked it
yourself!
After a tune-up to align the three
tuning capacitors, both sets were working really well. They are not brilliant
performers by superhet standards
but give the sort of performance one
would expect from a 5-valve TRF type
receiver.
RESURRECTION
RADIO
Valve Equipment Specialists
Repairs – Restoration – Sales
This rear view of chassis shows the power transformer cover (left) & the high
tension choke & capacitor block cover (right). Also shown is the type 45 output
valve at the end of the chassis.
The pair of C79s consumed a fair
amount of time and there were a few
worrying moments. However, they
are now back inside their elegant
cabinets and they look and sound
really good.
Cabinet restoration
My favourite is the AWA 45E. It
is an attractive looking set and Dick
has put a lot of time into restoring the
cabinet. In this instance, an entirely
new front panel has been made to
replace the original, which had a
broken fretwork.
The replacement panel has a lighter
coloured veneer than the original and
the two toned effect is most pleasing.
Dick does his own cabinet refinishing
and they get better with each one he
does.
So next time you’re at an auction
VALVES – 1200 types in stock
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matched $28 ea.
Parts are available for the enthusiast, including over 900 valve types, high voltage
cap­a citors, transformers, dial glasses,
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Circuit diagrams for most Australian makes
and models.
Send SAE for our catalog.
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Call in to our NEW showroom at:
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Phone: (03) 510 4486; Fax (03) 529 5639
where there are a few old radios going
under the hammer, say “Hello!” to
SC
Dick – he’s bound to be there!
Above: this massive old electrodynamic loudspeaker works
surprising­ly well for its age. The 7.5kΩ field coil is wired across the
high tension, not in series with it, as was the case in later years.
Left: the Radiola 45E in all its glory. This particular cabinet had
badly damaged fretwork & a new front panel has been made to
replace it. The lighter tone of the new panel looks even better in real
life than it does in a black & white photograph.
November 1994 73